Aquaculture is a fast-growing global industry. The conventional fisheries are declining dramatically and the seafood needed in the future will, to a greater extent, have to be produced by aquaculture.
However, in recent years, more challenges have arisen for this industry to be faced with. For the farming of salmonoids, this relates to, inter alia, the treatment of fish infected with crustaceous parasites like salmon louse, the escape of fish from floating farming cages, diseases due to viruses, bacteria and parasites, so-called “genetic contamination” of wild populations of salmonoids, and emissions of waste products from the cages into recipients. These challenges especially relate to the farming of fish in conventional floating cages allowing flow-through. Such open cages are constituted by a net which forms a closed enclosure for fish in particular, and which is kept afloat by means of a surrounding buoyancy system.
Previously, a device for farming aquatic organisms in a closed cage constituted by a tight, polymer material has been described, as in the patent document NO 175341, for example. The cage is provided with floating bodies. The patent document U.S. Pat. No. 4,798,168 discloses an alternative closed cage. The closed cage is provided with floating bodies and with a fence to prevent fish from jumping out of the cage. The patent document WO 2008/134842 discloses an open cage provided with a sectioned buoyancy system, in which the sections may include a submerged, symmetrical V-shaped portion. The patent document NO 167543 discloses a cage system with an inner cage and an outer cage which are kept afloat by a tubular, circular and sectioned buoyancy system. The patent document GB 21252.61 discloses a circular and sectioned buoyancy system, in which the sections are filled with polystyrene and foamed polyurethane.
Closed farming cages are known within the art. These are formed of a tight cloth material which forms a closed enclosure for aquatic organisms such as fish. To ensure that the water exchange rate is sufficient for maintaining a minimum oxygen level in the water within the cage, it is common to pump in so much water that the water surface inside the cage is higher than the water surface outside the cage. Thereby the pressure within the cage is larger than the ambient pressure and water will flow out of the cage through formed openings. This will subject the buoyancy system of a closed cage to larger forces than the buoyancy system of an open cage of the same size. In addition to keeping the actual cloth or net of the cage afloat, the buoyancy of the buoyancy system must be dimensioned for holding the amount of water within the cage that is above the water surface of the surrounding water. This water constitutes a considerable mass. In addition, this water has a moment of inertia which causes the wave influence on the buoyancy system to be greater than in an open cage where the wave motion passes substantially unobstructedly by the buoyancy system and into the cage.
The nets of open cages are attached to the buoyancy system by the buoyancy system being provided with projecting hooks. The buoyancy system may be provided with a handrail, and the projecting hooks may be fixed to the handrail. From so-called steel cages it is known to use special poles or supports provided with such hooks. The hooks may also be fixed to floating collars forming a buoyancy system.
Buoyancy systems for floating farming cages may be divided into two main groups, so-called steel cages and so-called plastic cages. Steel cages are made up of rectangular walkways of steel which are provided with floating bodies. The floating bodies may have the form of a rectangular box. The walkways are hinged together. The steel cages form a grid with longitudinal and transverse walkways. Each side of a square may be 10 m or 12 m long, for example. The seines, forming a closed enclosure, are placed in the grid and attached to the steel cage on hooks projecting from special poles or supports. Steel cages are also provided with railings, and the net may be attached to a handrail with hooks or lashings. The upper edge of the seine is thereby raised above the water surface and also forms a jump fence to prevent fish from escaping from the cage by jumping over the edge. The walkways of steel are formed with fixed lengths and cannot easily be adjusted in length. A plastic cage is constituted by at least one plastic tube which is welded together into a ring. Usually, the plastic cage is constituted by two concentric rings. Plastic cages with three concentric plastic rings are known as well. The plastic rings are connected to radially oriented damps of plastic or steel. Walkways may be placed on top of two concentric rings. The seine, which forms a closed enclosure, is placed within the innermost tube of the buoyancy system and is attached with projecting seine hooks. The seine hooks may be attached to the tube or to a railing projecting up from the buoyancy system. The circumference of the seine in a plastic cage may be, for example, between 90 m and 160 m, corresponding to a diameter of between approximately 30 m and 50 m. The plastic rings are formed with fixed lengths and cannot easily be adjusted in length.